Implants of this type allow bone material to grow into the open-celled surface structure, thereby assuring permanent fixation in the bone. A similar method is disclosed in DE-OS 27 30 004. It is particularly suggested therein that, for the creation of an uneven surface next to the positive pattern and ultimately next to the implant produced by the method as a metallic image of the pattern, particles in the form of small balls be adhered to the basic pattern. The implant has on the areas with uneven surfaces raised spherical segment-shaped elevations with back cuts. After implantation, bone material can grow in these surfaces and serve for permanent fixation of the implant into the bone.
Even if this may be successful in practice, the implants manufactured according to the stated procedure have the following disadvantages:
The growing of bone material is indeed possible, but medical research has shown boundary layer problems which cause malnourishment to the bone because of a missing substrate flow, so that lasting bony fixation of the implant becomes questionable. Also, the surface of the implant is still relatively smooth, whereby the primary stability of the implant right after the implantation is very low.
Behind the stated procedure lies the medical philosophy that it should be possible for an implant to be removed during corrective surgery which may become necessary, perhaps due to an infection, without destroying the bone that surrounds the implant. In the past years a new operation technique has been developed, which allows corrective surgery for the removal of the implant which is grown through with bone material (DE 32 24 265 A1), in which the bone is split, the implant removed, a new implant inserted, and the bone halves are rejoined and ultimately grow together again.
According to the above-cited reference it is suggested to make an implant according to the conventional lost positive technique, whereby an open-celled plastic molded body, e.g., a sponge, is used as a positive pattern. It is to be be observed in such foams that the strength of the webs enclosing the cells is a function of the cell width. This is a necessary consequence of the production of such foams and cannot be controlled. It has been found that the webs formed from use of such fine pieces are too thin, on account of which the above reference suggests strenthening of these webs by means of flowing wax or by means of a wax-water emulsion.
This layering procedure is relatively difficult to control. Additionally, despite a globally relatively constant form, the sponge also exhibits local peculiarities, such as smaller cells. This circumstance manifests itself in a negative manner during the casting process after the hardening of the poured metal into the negative cavity and upon removal of the ceramic mass. When one considers that the finished implants must have the highest purity in order to be implanted, it is extremely difficult to remove the last traces of the embedding mass of the cast, especially if by chance there are smaller cells in the spatial depths.
DE 90 11 363.2 U1 discloses another prosthetic part made according to the lost postive casting technique. As a positive pattern, a base with an adhered lattice element, preferably made of wax, is used. The lattice element is made by an injection molding process and is wrapped like a mat around the base. For this the lattice element must be elastic enough, or it will break. Due to this the webs of the lattice element have a preferred thickness of 0.05 cm. The webs of the lattice-shaped surface of the implant will then of course have a comparable order of magnitude after completion of the molding. If the webs must have a greater thickness, then a layering procedure of the type described above must again be considered.
Additional procedures are described in DE 39 17 033 C1 and DE 39 28 394 C2 (U.S. Pat. Nos. 5,042,560 and 5,016,702, respectively) of the present inventor. Therein it is suggested that the webs of an open-celled plastic substrate (sponge), which is formed on a wax base and creates together therewith the positive pattern of the molded piece (implant) to be produced, are to be strengthened by a layer of a two-component silicone (U.S. Pat. No. 5,042,560) or a poly(methyl methacrylate) or polyester resin (U.S. Pat. No. 5,106,702).
An object of the present invention is to develop a process which improves the above-cited art so that the surface structures of the implant created by the process are discrete and so stable, that an additional treatment of the positive pattern is not necessary, and that difficulties do not appear upon removal of the ceramic embedding mass after the molding process because of local deviations of the structures.